The kinetic patterns of fresh, frozen-thawed, or cultured human leukocytes are studied by tagging the cells ex vivo with 111-Indium, a radioisotopic label, and measuring their distribution throughout the body by means of gamma camera imaging and gamma counting of blood samples. This type of study has widespread applications. The most common use of these radiolabeled leukocyte trafficking studies is for abscess localization in cases of suspected infection not definitively diagnosed by other non-invasive studies. In these cases autologous or allogeneic granulocytes, collected by simple phlebotomy or by apheresis, are labeled with 50 uCi of 111-Indium per kg of patient weight, not to exceed 500 uCi total. Twenty-two studies for the purpose of abscess localization were performed in Clinical Center patients enrolled in a variety of protocols in the past 7 years. Three of these 22 were positive, and led to appropriate therapy for a case of salmonella osteomyelitis in a hemophiliac patient with human immunodeficiency virus infection, staphylococcal pneumonia in a patient with aplastic anemia, and diverticular abscess in a patient with Zollinger-Ellison syndrome. In many of the negative studies, unnecessary surgical exploration was avoided. Three of the 22 indium-labeled allogeneic leukocyte studies were performed in patients with chronic granulomatous disease, in one case to document clearance of pulmonary infection, in the second to confirm lack of trafficking in an alloimmunized recipient, and in the third to localize a cause for fever. In all three cases, gamma imaging results were used to guide therapy, specifically, to terminate a course of granulocyte transfusions when they were either no longer necessary or were ineffective, or to prevent an exploratory laparotomy.Collaborative trials with the Surgery Branch, NCI, have investigated the diagnostic utility and prognostic application of radiolabeled autologous tumor infiltrating lymphocyte (TIL) studies in patients with metastatic melanoma. TIL trafficking studies revealed metastatic deposits that were undetected clinically, and TIL trafficking to sites of tumor was strongly correlated with tumor regression following TIL infusion. Six studies of indium-labeled autologous cloned T cells with antimelanoma activity revealed a trafficking pattern different from that of TIL cells and further studies of these cloned cells, particularly in patients with disease in imageable sites, are in progress.